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THE UNDERSTORY: Phosphorus and Australian plants

By Susan Starr.

This is the first in a series of occasional columns providing further information from the scientific literature on topics addressed by SDHS speakers.

Our January speaker, Jo O’Connell, referred to the well-known intolerance of Australian natives to phosphorus. Actually, it is not that they cannot tolerate phosphorus but, as she pointed out, that they have evolved very efficient methods for absorbing what little phosphorus is available in the phosphorus poor soils typical of Australia. Thus providing additional phosphorus is not necessary and may create chemical imbalances.

The soils of Western Australia are among the poorest on the planet, and some of these soils tightly bind phosphate, making what little there is difficult for plants to use. According to the literature, to adapt to these circumstances many Australian plants have evolved mechanisms, such as specialized root structures, that increase their uptake of phosphorus; they may also be able to distribute phosphorus from older to younger tissues. *1 As a result, Australian plants frequently will grow and thrive with less phosphorus than North American plants of similar size and leaf structure. At the same time, many Australian plants are relatively iron inefficient; they are used to growing in soils with an abundant supply of iron. When phosphorus is applied to these plants they readily absorb it, creating an excess of phosphorus. The excess phosphorus binds with iron, in turn creating an insoluble phosphate. The supply of available iron, on which the poor plant is counting, is thereby depleted, and the result is iron deficiency. This is why adding fertilizers containing phosphorus can lead to poor growth, chlorosis (yellowing leaves), leaf drop and susceptibility to fungi such as phytophthora. Additional iron can sometimes alleviate these symptoms.

The good news? Not all Australian plants are sensitive to additional phosphorus. In fact, a study by the Society for Growing Australian Plants (SGAP), found that of over 800 species studied, 82% were not particularly phosphorus sensitive. You can find their list of Australian natives, grouped by relative phosphorus sensitivity, at APOL8/dec97-4.html. Not surprisingly, plants widely planted in San Diego, such as Dodonea viscosa (hop bush), most of the Callistemons (bottlebrushes), Agonis flexuosa (peppermint tree), and Boronia denticulate are relatively insensitive. However, what I found most interesting about this list was that different species of the same genus had different sensitivities to phosphorus. So, for example, Banksia audax, elderana, laevigata, lanata, littoralis, menziesii, petiolaris, and speciose are all relatively insensitive, but Banksia aculeata, canei, cunninghamii, grandis, and victoriae exhibited high rates of toxicity at high levels of additional phosphorus. Bottom line: if you’re having trouble with Australian natives, you may want to check out the SGAP list. Perhaps there’s another variety or a different genus better suited to the soil in your San Diego garden.

Susan Starr is SDHS Newsletter editor and a former science librarian.

1 Leake, Simon. Phosphorus and Iron Nutrition in Australian Native Plants. Australian Plants Online, The Society for Australian Plants, 1993.

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